Conventionally, there is known an illumination device provided with a light guide plate which guides light emitted from a light source unit and emits the light outward (see, e.g., Japanese Unexamined Patent Application Publication No. 2011-3367). The light guide plate includes, for example, fine concave pattern marks (dots) disposed in a matrix shape on the opposite main surfaces (a front surface and a rear surface) thereof. The light is emitted outward through the dots. The dots are arranged in, for example, a square lattice pattern. The dots on the front surface are offset with respect to the dots on the rear surface in the arrangement direction of the square lattice. By disposing the dots in this way, the number of bright spots formed by the dots is increased, so that the light guide plate can uniformly emit light.
However, in the aforementioned illumination device, the dots on the front surface and the dots on the rear surface interfere with each other. Thus, when the light guide plate is viewed from the front surface side or the rear surface side, a periodic pattern of stripe-shape (moire) may be generated in some cases.
As one means for removing the moire, it is thinkable that the interference between the dots on the front surface and the dots on the rear surface is reduced by bonding light diffusion sheets to the front surface and the rear surface of the light guide plate or by adding a light diffusion property to a region where the dots are not formed. However, the addition of the light diffusion property makes it difficult to control the distribution of the light emitted from the light guide plate. Particularly, when the illumination device is turned off, the opacity of the light guide plate is noticeable and the appearance of the light guide plate is poor.